By Prince McLean

Published: 07:00 AM EST

While Apple's share of the entire US PC market hovers between 6% and 8% depending on the source, its share of the 802.11n WiFi base station market is even higher.

Stephen Baker, an analyst for market research firm NPD Group, told AppleInsider that Apple took 10.6% of the market in unit volume last month. He added that the company's revenue and profit share on sales of the routers are even higher.

Last year, Apple noted on its website that the AirPort Extreme was ranked by NPD as the top selling 802.11n router. While Apple no longer advertises that, Baker said that the AirPort Extreme has been the top selling 802.11n router for five of the last nine months.

Last week, the analyst told Macworld that the AirPort Extreme lead US retail sales as the top selling router in April, while the new Time Capsule topped sales as the most popular Network Attached Storage device. Despite their overlapping functionality, it was reported that strong sales of Time Capsule were augmenting sales of the AirPort Extreme base station rather than cannibalizing them.

Combined with sales of the compact AirPort Express, which was upgraded to support the faster 802.11n standard in March, Apple took fourth place in overall 802.11n base station sales, behind Cisco's Linksys brand, D-Link, and Netgear.

Apple markets its AirPort base station line to users of both Macs and Windows, which allows it to sell the product beyond its own user base, following the same cross platform strategy of the iPod, iTunes, QuickTime, and the iPhone. Linksys, D-Link, and Netgear also advertise Mac compatibility, but their products do not always deliver flawless support for Safari on the Mac. That helps give Apple a home field advantage in selling to Mac users.

Additionally, Apple's retail and online stores are selling AirPort base stations to new Mac users without any competition. "This stuff is just flying off the shelf in the Apple stores," Baker told Macworld. "They don’t get nearly enough credit for the value proposition that the stores bring."

On the Windows PC side, Apple still faces formidable competition. Baker told AppleInsider that Linksys "has recently delivered a number of new SKUs in the 802.11n segment that have done very well driving their volume." Several years ago, Linksys began using Linux-based software in its wireless routers, a move that compelled it to publicly release its source code under the GPL. That availability enabled Linux users to add previously restricted, high end router software features to low cost Linksys base stations, as well allowing Linksys' competitors to use its router software to compete against it with their own hardware. Linksys has since moved to using the proprietary VxWorks kernel in its flagship router products.

Apple's AirPort line also uses proprietary software, in addition to custom Mac and Windows client software for configuration rather than using a webpage interface as most base stations do. That may limit the appeal of its AirPort line among some Windows users, but it also allows Apple to install support for unique features that differentiate the AirPort line, including Bonjour automatic printer sharing and AirPort shared disk discovery as well as AirTunes audio streaming support from iTunes to an AirPort Express or Apple TV.

Brisk base station sales suggest that Apple's "fourth leg" behind its Mac, iPod and iTunes business, and the iPhone is not the emerging Apple TV but rather AirPort, a business segment that has performed well, albeit almost invisibly, since its introduction back in 1999, two years before the iPod.